There are gamified rules put in by the gods essentially. If it is surrounded by walls, it doesn't count as an active core. He has backup cores buried in the maze currently, but they would have to open a wall before they could castle to it. Likewise, if you did something like lock one behind a door and destroy the key, the door would become fragile and they wouldn't be able to use points to repair it. You could put your core in a wall with no doors, but you would lose access to the dungeon, and have to re-buy the real estate all over again once you opened it up.